Emission current regulated power supply for thermionic filament



May 16, 967 R. L. WATTERS 3,320,474

EMISSION CURRENT REGULATED POWER SUPPLY FOR THERMIONIC FILAMENT Filed Nov. 1, 1963 E 3/ -!00 Gauge //7 venfor Robe/2 L. Waffers United States Patent Ofiice 3,320,474 Patented May 16, 1967 3,320,474 EMISSHON CURRENT REGULATED POWER SUIPFPLY FOR THERMIONIC FELAMENT Robert L. Wattcrs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 1, 1963. Ser. No. 320,629 6 Claims. (Cl. 315-94) This invention generally relates to power supplies for the filament of electron emitting devices and more particularly pertains to a power supply for this service having a control circuit which ensures a constant supply of electrons in the device.

It is desirable oftentimes to provide a source of electrons, within an electronic device, that maintains a substantially constant rate of electron emission. Sometimes a constant rate of electron emission must be maintained in the presence of pressure fluctuations within the device that tend to disturb the rate of electron emission. An example of an electronic device wherein a substantially constant rate of electron emission is advantageously maintained in the presence of pressure fluctuations is found in the ion gauge. The heart of an ion gauge comprises one or more electrodes so positioned and biased that electron emission causes the ionization of ionizable material, such as gases. The resulting ions are collected to provide an indication of the quantity of ionizable material in the vicinity of the electron emitter.

Currently, ion gauges are frequently used to indicate the pressure in evacuated systems. In such an application, provision of a constant rate of electron emission within the ion gauge ensures a constant rate of ion production and therefore considerably simplifies analysis of the data received.

Many complex and costly equipments and techniques are known which are capable of providing the necessary filament power regulation in response to electronic emission from the filament. However, in view of the increasing popularity of the ion gauge as a useful device in every day commercial endeavors it would be highly desirable to provide a simple, efficient and inexpensive emission regulated power supply for ion gauge filaments. The regulator should compensate for both line voltage variations and effects within the ion gauge tube, both of which tend to vary the rate of electron emission from the filament.

Accordingly, it'is a primary object of this invention to provide an eflicient, relatively inexpensive, regulated power supply for ion gauge filaments, using as few components as possible.

It is another object of this invention to provide a relatively inexpensive filament power supply for ion gauges which compensates for line voltage variations and conditions within the ion gauge tube which would otherwise cause changes in the rate of electron emission.

Briefly, in accordance with the present invention, an emission current regulated power supply is provided requiring only one silicon controlled rectifier and one unijunction transistor as the sole active elements. The silicon controlled rectifiers primary electrodes are connected in series with a source of alternating current power and the ion gauge filament. The unijunction transistor is connected to the control electrode of the silicon controlled rectifier and determines the relative time in the alternating current cycle at which the silicon controlled rectifier fires and supplies power to the filament. In turn, the charging rate of a capacitor in the emitter circuit of the unijunction transistor determines the firing time of this device. I provide at least one discharging and two charging circuits for the capacitor. The discharge circuit ensures that the voltage across the capacitor essentially follows the voltage of the source from about zero voltage down to the negative-most portion of its cycle, providing substantial line voltage compensation. One charging circuit provides charging at a predetermined rate and the other charging circuit provides a charging current which varies in accordance with electron emission to compensate for changes in conditions within the ion gauge. Preferably, a third charging circuit is included which provides charging of the capacitor at a rate which varies in accordance with the magnitude of the voltage from the source, resulting in improved line voltage compensation.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which The single figure is a schematic circuit diagram of a preferred embodiment of my invention.

In the figure, power supply 1 includes a pair of input terminals 2 and 3 which are adapted to be connected to a source of alternating current electric power to provide energization of primary winding 4 of transformer 5. Terminals 2 and 3 may be connected conveniently to a source of readily available commercial power such as the standard volt 60 cycle per second power lines. When terminals 2 and 3 are connected to a source of alternating current power, the secondary Winding of transformer 5 is energized in a well-known manner and may be considered, itself, to be the source of alternating current power for power supply 1. Therefore, in the interest of simplicity and clarity, the transformer secondary winding will be considered hereinafter and in the appended claims to be the source 6 of alternating current power and will be so designated. Of course, it is not required that the source of alternating current power used in the practice of my invention be a transformer secondary winding since other equivalent means may be substituted therefor, such as the output terminals of an alternating current generator or inverter, in order to more readily meet the requirements of particular applications.

As illustrated in the drawing, silicon controlled rectifier (SCR) '7 and output means adapted to be connected to a filament, such as terminals 8 and 9, are connected in series circuit relationship with source 6 of alternating current power to provide a first series loop network. Anode 10 of SCR 7 is electrically connected to one extremity, or terminal, of source 6 and cathode 11 of SCR 7 is electrically connected to terminal 8. Terminal 9 of the output means is connected to the other extremity, or terminal, of course 6.

Control electrode 12 of SCR 7 is electrically connected to the first base 13 of unijunction transistor (UJT) 14 and the second base 15, of UJT 14, is electrically connected to anode 10 of SCR 7. Resistor 16 is connected from control electrode 12 and first base 13 to cathode 11, and emitter 17 of UJT 14 is connected by capacitive means 18, to cathode 11.

The operation of the circuit set forth thus far is wellknown to those skilled in the art and is described in a plurality of publications. One such publication is the Silicon Controlled Rectifier Manual, second edition, published by the assignee of the present invention. While the circuit operation will be described herein, a more detailed explanation of the basic cooperation between and .function of the SCR and U] T, in controlling the amount of power delivered to a power-consuming device, or load, may be found in the aforementioned publication, particularly commencing at page 44 therein.

Briefly, operation of the circuit thus far described is as follows. When the voltage of emitter 17 is caused to rise to a magnitude which is a predetermined fraction, approximately one half of the voltage difference between the voltage of second base 15 and the voltage of first base 13, there is a sharp drop in the resistance between emitter 17 and first base 13. The voltage across capacitor 18, and hence the charging rate thereof, determines the time required for emitter 17 to reach this predetermined voltage. When this voltage is reached a sudden increase in current through resistor 16 occurs, as a result of capacitor 18 supplying energy through the suddenly decreased resistance between emitter 17 and first base 13 to resistor 16, and a voltage pulse is applied to control electrode 12, turning SCR 17 on, or changing the resistance between rectifier anode and rectifier cathode 11 from a relatively high magnitude to a relatively low magnitude. Source 6 supplies power through the relatively low series resistance of SCR 7 to terminals '8 and 9 and the flow of power continues during the remainder of the one half cycle. SCR 7 becomes nonconductive during the alternate half cycle, since it is an asymmetrically conducting device.

When the charging rate of capacitor 18 is rapid, firing of SCR 7 occurs early in the half-cycle during which SCR 7 may become conductive and maximum power is available at terminals 8 and 9. Conversely, when capacitor 18 charges relatively slowly, SCR 7 fires later in the power cycle and a reduced amount of power is available at terminals 8 and 9. In this way, the charging rate of capacitor 18 determines the amount of power which is supplied during any given half-cycle by source 6 to any power absorbing means, as a filament, connected between terminals 8 and 9.

In accordance with the present invention a discharge circuit for capacitor 18 is provided that efiects a voltage across capacitor 18 substantially equal in magnitude to the voltage output of source 6 at the negative-most portion of its alternating cycle. The word discharged is used herein algebraically and charging of capacitor 18 in the opposite direction from that required to fire UJT 14 is referred to as further discharging of capacitor 18. The portion of the cycle under consideration is that which occurs when the voltage output from source 6 is at a maximum and the extremity connected to terminal 9 is positive with respect to the extremity of source 6 connected to rectifier anode 10. The means for achieving discharge of capacitorlS and effecting the aforementioned voltage relationship may, conveniently, take the form of asymmetrically conducting device 19. Cathode 20, of device 19, is connected to rectifier anode 10 and anode 21, of device 19, is connected to emitter 17, of UJT 14. Asymmetrically conductive device 19 becomes heavily conductive during the alternate, or negative portion, of the power cycle and effects a rapid discharge of capacitor 18 through the normally low impedance power absorbing means connected to terminals 8 and 9.

By discharging capacitor 18 to a voltage substantially equal to that at the negative-most portion of the voltage cycle from source 6, compensation is achieved for variations which may occur in the effective, or root mean square (R.M.S.), voltage supplied by source 6. The compensation is achieved because the negative and positive alternations of source 6 are substantially equal and firing of SCR 7 during the positive portion of the cycle is delayed for a longer portion of the cycle when the R.M.S. voltage is greater because capacitor 18 is discharged to a greater extent, resulting in a longer charging time for any predetermined charging rate. Conversely, a lesser discharge of capacitor 18 and faster firing time relative to the positive portion of the cycle attends a decrease in efiective voltage. Thus, when the effective voltage of source 6 increases and a greater quantity of power would be supplied to a power absorbing device, or load, connected to terminals 8 and 9, the firing is delayed so that conduction of SCR 7 occurs later in the cycle, providing compensation. Conversely, the firing is more rapid when the effective voltage of source 6 is reduced to avoid a decrease in power available at terminals 8 and 9.

A charging circuit is connected to capacitor 18 that is arranged to provide a predetermined charging rate therefor. The charging circuit includes a series loop network constituted of a source 22 of unidirectional voltage, a first resistor 23 and a second resistor 24. Resistors 23 and 24 are connected in series across source 22. A resistor 25 is connected from the junction of resistors 23 and 24 to emitter 17 of UJT 14. The resistance values are preferably, but not necessarily, selected so that the voltage difference between opposite ends of resistor 24 is about equal to the desired ion collector voltage of a particular ion gauge to be used with the regulator. In this way, the ion collector may be operated at ground potential, for simplicity of current sensing, and yet maintain the required negative potential with respect to an electron emitter connected to terminals 8 and 9.

The juncture between resistor 23 and source 22 is connected to output means, as terminal 26, adapted to be connected to the electron collector of an ion gauge. Source 22 is poled, or inserted in the proper polarity relationship, to bias the electron collector positively with respect to the filament. The voltage magnitude of source 22 is selected to provide the proper bias potential for the electron collector to ensure temperature limited, as 0pposed to space charge limited, current emission from the filament. Themperature limited emission is required in ion gauge tubes.

A variable resistor 27, that is shunted 'by a filter capaci tor 28, is connected from the juncture of resistor 24 and source 22 to cathode 11 of SCR 7. Thus, the voltage across variable resistor 27 varies in magnitude in accordance with the ion gauge electron emission current. The polarity of the voltage occurring across resistor 27, as a result of emission current received at input means 26, is such as to oppose charging of capacitor 18 by the aforementioned charging circuit. Thus, resistor 27 is arranged to be responsive to variations in the electron current from the electron collector to provide opposite variations in the predetermined charging rate of capacitor 18. Resistor 27 is variable in order to set the emission current at a particular desired gate. I have found that the emission current may be varied over at least two decades (factor of by adjusting the resistance value of variable resistor 27, and yet maintain proper regulation of emission current.

In some applications the circuit of FIGURE 1 provides over-compensation for variations in the voltage of source 6. That is to say, emission current varies inversely as variations in source voltage. The over-compensation does not occur to a great degree and may be tolerated in most applications where it occurs. The overcompensation may not be corrected readily by altering the values of the resistive components or the value of source 22 in the above-described charging circuits, while yet maintaining the desired wide range of adjustment described in connection with variable resistor 27. Therefore, in the preferred embodiment of my invention I provide additional charging means for capacitor 18 that is adapted to provide an additional charging current therefor which varies in magnitude in the same direction as variations in the effective voltage of source 6. In FIG- URE 1 the additional charging means takes the form of resistor 29 connected from second base 15 of UJT 14 to emitter 17 thereof. By properly selecting the resistance value of resistor 29 I have achieved compensation for approximately 30% variations above and below the normal voltage of source 6 with only a 2% variation in emission current (from 0.98 to 1.00 milliampere). The selected resistance value provided an impedence to current through resistor 29 which was about double the impedance from emitter 17 to ground through resistors 23, 24 and 25.

Power supply '1 is arranged for operation in an ion gauge system by connecting the filament 30 of an ion gauge, such as 31, across terminals 8 and 9 to be supplied po'wer therefrom. The electron collector 32 of tube 31 is electrically connected to terminal 26. Thereafter, terminals 2 and 3 are connected to a suitable source of alternating current power and resistor 27 varied in resistance magnitude to achieve a particular desired rate of electron emission from filament 30. The emission current may be monitored, conveniently, by providing an ammeter (not shown) in series with the conductive means which connect electron collector 32 to terminal 26.

Variations in the emission current are rapidly corrected by the influence of voltage changes across variable resistor 27 upon the predetermined charging rate established by the network including resistors 23, 24 and 25, and source 22. Compensation for variations in the voltage supplied by source 6 is provided by the capacitor discharging circuit, including asymmetrically conducting device 19. Any slight over-compensation is corrected by the additional charging circuit including resistor 29.

By way of aiding those skilled in the art in the practice of my invention, and not to be construed in limiting sense, the following specific components, connected as shown in FIGURE 1, provide a particularly desirable filament power supply for use in accordance with my invention:

SCR 7-Type CU UJT 14Type 2N1671A Diode l9-Type 1Nl692 Transformer 5-Merit type P3088 Resistor 16--100 ohms Resistor 23-62,000 ohms Resistor 2433,000 ohms Resistor 25l 00,000 ohms Resistor 29220,000 ohms Resistor 27250,000 ohms, variable Capacitor 18-.033 mfd. Capacitor 281.0 m'fd.

Source 22150 volts The above-mentioned specific power supply was used with a type 5966 ion gauge tube. When resistor 27 was adjusted to 200,000 ohms, 30,000 ohms, and 4,000 ohms, the emission current was substantially constant at .1, 1, and 10 milliamperes, respectively. With the 1 milliampere setting, variations in input voltage to terminals 2 and 3 from 75 through 135 volts caused only a 2% variation in the emission current. Comparable regulation was observed with the other two settings. Thus, the relatively simple and inexpensive power supply of FIGURE 1 provided more than adequate emission control and compensation for line voltage variations. The time constant for the charging circuit including resistors 23, 24 and 25 was approximately 4 milliseconds.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. For example, resistor 25 may be eliminated by increasing the resistance values of resistors 23 and 24 to maintain the resistance to ground from emitter 17 constant. Adso, improved regulation can be achieved by reducing the value of capacitance of capacitor 18 and increasing the resistance values of resistors 23, 24, 25 and 29 by the same percentage to preserve the same time constant (RC) for the charging circuits. I do not prefer these expedients because the component tolerances become more critical and less susceptible of mass production. However, it is to be understood that the appended claims are intended to cover all such modifications and changes as these, and others that fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a filament power supply for use with a device having a thermionic electron emissive filament and a current collector electrode for collecting substantially all of the electrons from said filament; said power supply including output means coupled to said filament, a source of alternating current power and a silicon controlled rectifier having anode, cathode, and control electrodes; said output means, source, rectifier anode and rectifier cathode being electrically connected in a series loop network; the improvement of control means for said silicon controlled rectifier comprising:

(a) a unijunction transistor having first and second base electrodes and an emitter electrode;

(b) conductive means electrically connecting said second base electrode to said rectifier anode and electrically connecting said first base electrode to said control electrode;

(c) resistive means electrically connected from said first base electrode to said rectifier cathode and capacitive means connected from said emitter electrode to said rectifier cathode;

(d) circuit means coupled to said capacitance means for effecting a voltage across said capactive means that is substantially equal in magnitude to the volt age output of said alternating current source at the negative-most portion of the voltage cycle of said source;

(e) a charging circuit connected to said capacitive means and providing a predetermined charging rate therefor; and,

(f) means coupling said electron emissive filament to said charging circuit and responsive to variations in the electron current therefrom to provide an opposite variation in said predetermined charging rate.

2. The control means of claim 1 including additional charging means coupled to said capacitive means and providing an additional charging current which varies in magnitude in the same direction as variations in the effective voltage of said source.

3. The control means of claim 1 wherein the means for effecting a voltage across said capacitive means that is substantially equal in magnitude to the voltage output of said alternating current source at the negative-most portion of the voltage cycle of said source comprises an asymmetrically conducting device having an anode and a cathode, said cathode being connected to said rectifier anode and said anode being connected to said emitter.

4. The control means of claim 1 wherein said charging circuit comprises a series loop network including a source of unidirectional voltage, a first resistor and a second resistor and a resistor connected from the juncture of said first and second resistor to said emitter; and said means connected to said electron emissive filament and responsive to variations in the electron current therefrom to provide an opposite variation in said predetermined charging rate comprises a resistor and a shunt capacitor therefor connected from the juncture of said second resistor and said source of unidirectional voltage to said rectifier cathode electrode; said control means further including input means connecting said electron collector to the juncture of said first resistor and said source of unidirectional voltage, said source of unidirectional voltage being poled to positively bias said electron collector relative to said filament.

5. In a filament power supply for use with a device having a thermionic electron emissive filament and a current collector electrode for collecting substantially all of the electrons from said filmament; said power supply including output means coupled to said filament, a source of alternating current power and a silicon controlled rectifier having anode, cathode and control electrodes; said output means, source, rectifier anode and rectifier cathode being electrically connected in a series loop network; the improvement of control means for said silicon controlled rectifier comprising:

(a) a unijunction transistor having first and second base electrodes and an emitter electrode;

(b) conductive means electrically connecting said second base electrode to said rectifier anode and electrically connecting said first base electrode to said control electrode;

(0) a resistor connected from said first base electrode to said rectifier cathode and a capacitor connected from said emitter electrode to said rectifier cathode;

(d) an asymmetrically conducting device having an anode and cathode, said cathode being connected to said rectifier anode and said anode being connected to said emitter;

(e) a series loop network including a source of unidirectional voltage, a first resistor and a second re- 15 sistor; (f) means coupling said first and second resistors to said emitter electrode; and

(g) a resistor and a capacitor connected from the juncture of said second resistor and said source of unidirectional voltage to said rectifier cathode, and input means connecting said electron collector to the juncture of said first resistor and said source of unidirectional voltage, said source of unidirectional voltage being poled to positively bias said electron col lector relative to said filament.

6. The control means of claim 4 wherein a resistor is 10 connected from said rectifier anode to said emitter.

References ited by the Examiner UNITED STATES PATENTS 3,235,711 2/1966 Bergen et al 30788.5

JOHN W. HUCKERT, Primary Examiner.

J. D. CRAIG, Assistant Examiner. 

1. IN A FILAMENT POWER SUPPLY FOR USE WITH A DEVICE HAVING A THERMIONIC ELECTRON EMISSIVE FILAMENT AND A CURRENT COLLECTOR ELECTRODE FOR COLLECTING SUBSTANTIALLY ALL OF THE ELECTRONS FROM SAID FILAMENT; SAID POWER SUPPLY INCLUDING OUTPUT MEANS COUPLED TO SAID FILAMENT, A SOURCE OF ALTERNATING CURRENT POWER AND A SILICON CONTROLLED RECTIFIER HAVING ANODE, CATHODE, AND CONTROL ELECTRODES; SAID OUTPUT MEANS, SOURCE, RECTIFIER ANODE AND RECTIFIER CATHODE BEING ELECTRICALLY CONNECTED IN A SERIES LOOP NETWORK; THE IMPROVEMENT OF CONTROL MEANS FOR SAID SILICON CONTROLLED RECTIFIER COMPRISING: (A) A UNIJUNCTION TRANSISTOR HAVING FIRST AND SECOND BASE ELECTRODES AND AN EMITTER ELECTRODE; (B) CONDUCTIVE MEANS ELECTRICALLY CONNECTING SAID SECOND BASE ELECTRODE TO SAID RECTIFIER ANODE AND ELECTRICALLY CONNECTING SAID FIRST BASE ELECTRODE TO SAID CONTROL ELECTRODE; (C) RESISTIVE MEANS ELECTRICALLY CONNECTED FROM SAID FIRST BASE ELECTRODE TO SAID RECTIFIER CATHODE AND CAPACITIVE MEANS CONNECTED FROM SAID EMITTER ELECTRODE TO SAID RECTIFIER CATHODE; (D) CIRCUIT MEANS COUPLED TO SAID CAPACITANCE MEANS FOR EFFECTING A VOLTAGE ACROSS SAID CAPACITIVE MEANS THAT IS SUBSTANTIALLY EQUAL IN MAGNITUDE TO THE VOLTAGE OUTPUT OF SAID ALTERNATING CURRENT SOURCE AT THE NEGATIVE-MOST PORTION OF THE VOLTAGE CYCLE OF SAID SOURCE; (E) A CHARGING CIRCUIT CONNECTED TO SAID CAPACITIVE MEANS AND PROVIDING A PREDETERMINED CHARGING RATE THEREFOR; AND, (F) MEANS COUPLING SAID ELECTRON EMISSIVE FILAMENT TO SAID CHARGING CIRCUIT AND RESPONSIVE TO VARIATIONS IN THE ELECTRON CURRENT THEREFROM TO PROVIDE AN OPPOSITE VARIATION IN SAID PREDETERMINED CHARGING RATE. 